Security system for swimming pools and like bodies of water

Abstract

An apparatus for detecting a person in a body of water includes a transmitter for generating an electrical swept frequency signal having a frequency that continuously varies between upper and lower limits, at least one pair of transducers disposed in a body of water including a transmitting transducer connected to receive the swept frequency signal for launching a beam of acoustical waves in response to the swept frequency signal in the body of water and a receiving transducer disposed opposite the transmitting transducer for receiving the acoustical waves and for converting received acoustical waves into an electrical received signal, and a receiver connected to the receiving transducer for producing a detected signal in response to the received signal, for producing a threshold signal in response to the detected signal, and for initiating an alarm when the detected signal falls below the threshold for at least a predetermined length of time.

I claim: 
     
       1. An apparatus for detecting a person in a body of water comprising: means for generating an electrical swept frequency signal having a frequency that continuously varies between upper and lower limits;   at least one pair of transducers disposed in a body of water including a transmitting transducer connected to receive the swept frequency signal for launching a beam of acoustical waves in response to the swept frequency signal in the body of water and a receiving transducer disposed opposite the transmitting transducer for receiving the acoustical waves and for converting received acoustical waves into an electrical received signal; and   means connected to the receiving transducer for producing a detected signal in response to the received signal and for initiating an alarm when the detected signal falls below a threshold for at least a predetermined length of time.   
     
     
       2. The apparatus of claim 1 including four pairs of transducers, one of each pair of transducers being located at each of four spaced apart locations, two receiving transducers being located adjacent each of two of the locations and two transmitting transducers being located adjacent each of the other two locations. 
     
     
       3. The apparatus of claim 1 wherein the means for generating comprises a first oscillator for generating a modulating signal having a first frequency and a variable magnitude and a second oscillator connected to the first oscillator for generating a variable frequency signal having a frequency varying in response to the magnitude of the modulating signal as the swept frequency signal wherein the variable frequency signal has a higher frequency than the first frequency. 
     
     
       4. The apparatus of claim 3 wherein the means for generating comprises a bandpass filter receiving the variable frequency signal for eliminating undesired frequency components from the variable frequency signal and thereby producing the swept frequency signal. 
     
     
       5. The apparatus of claim 1 wherein the means for initiating comprises means for comparing the detected signal to the threshold and for generating a reset signal pulse while the detected signal exceeds the threshold and means for detecting an interruption in reset signal pulses. 
     
     
       6. The apparatus of claim 5 including means for generating the threshold comprising a rectifier receiving the detected signal and producing a rectified detected signal and a low pass filter receiving the rectified detected signal and producing the threshold. 
     
     
       7. The apparatus of claim 5 wherein the means for comparing comprises a comparator receiving the threshold and the detected signal. 
     
     
       8. The apparatus of claim 5 wherein the means for detecting an interruption in the pulsed reset signal pulses comprises a timer for timing an interval between consecutive reset signal pulses. 
     
     
       9. The apparatus of claim 1 wherein the means for producing includes calibration means for indicating whether the detected signal is within a predetermined range and a variable gain amplifier for adjusting the detected signal to the predetermined range. 
     
     
       10. The apparatus of claim 9 wherein the calibration means comprises: means for rectifying the detected signal;   a low pass filter for filtering the rectified detected signal and thereby producing a DC signal;   a first comparator receiving the DC signal and a first calibration threshold for determining whether the DC signal exceeds the first calibration threshold;   a second comparator receiving the DC signal and a second calibration threshold for determining whether the DC signal is less than the second calibration threshold; and   means connected to the first and second comparators for indicating whether the DC signal lies between the first and second calibration thresholds.   
     
     
       11. An apparatus for detecting a person in a body of water comprising: means for generating an electrical transmission signal;   at least one pair of transducers disposed in a body of water including a transmitting transducer connected to receive the electrical transmission signal for launching a beam of acoustical waves in response to the electrical transmission signal in the body of water and a receiving transducer disposed opposite the transmitting transducer for receiving the acoustical waves and for converting received acoustical waves into an electrical received signal; and   means connected to the receiving transducer for producing a detected signal in response to the received signal, for producing a threshold signal in response to and determined by the detected signal, and for initiating an alarm when the detected signal falls below the threshold signal for at least a predetermined length of time.   
     
     
       12. The apparatus of claim 11 including four pairs of transducers, one pair of transducers being located adjacent each of four different locations, two transducers located adjacent each of two of the four locations being receiving transducers and two transducers located adjacent each of the other two of the four locations being transmitting transducers. 
     
     
       13. The apparatus of claim 11 wherein the means for generating comprises a first oscillator for generating a modulating signal having a first frequency and a variable magnitude and a second oscillator connected to the first oscillator for generating a variable frequency signal having a frequency varying in response to the magnitude of the modulating signal as a swept frequency signal wherein the variable frequency signal has a higher frequency than the first frequency. 
     
     
       14. The apparatus of claim 13 wherein the means for generating comprises a bandpass filter receiving the variable frequency signal for eliminating undesired frequency components from the variable frequency signal and thereby producing the swept frequency signal. 
     
     
       15. The apparatus of claim 11 wherein the means for producing the threshold signal comprises a rectifier receiving the detected signal and producing a rectified detected signal and a low pass filter receiving the rectified detected signal and producing the threshold signal. 
     
     
       16. The apparatus of claim 11 wherein the means for initiating comprises means for comparing the detected signal to the threshold signal and for generating a reset signal pulse while the detected signal exceeds the threshold signal and means for detecting an interruption in reset signal pulses. 
     
     
       17. The apparatus of claim 16 wherein the means for detecting an interruption in the pulsed reset signal pulses comprises a timer for timing an interval between consecutive reset signal pulses. 
     
     
       18. The apparatus of claim 16 wherein the means for comparing comprises a comparator receiving the threshold signal and the detected signal. 
     
     
       19. The apparatus of claim 11 wherein the means for producing a detected signal includes calibration means for indicating whether the detected signal is within a predetermined range and a variable gain amplifier for adjusting the detected signal to the predetermined range. 
     
     
       20. The apparatus of claim 19 wherein the calibration means comprises: means for rectifying the detected signal;   a low pass filter for filtering the rectified detected signal and thereby producing a DC signal;   a first comparator receiving the DC signal and a first calibration threshold for determining whether the DC signal exceeds the first calibration threshold;   a second comparator receiving the DC signal and a second calibration threshold for determining whether the DC signal is less than the second calibration threshold; and   means connected to the first and second comparators for indicating whether the DC signal lies between the first and second calibration thresholds.   
     
     
       21. An apparatus for detecting a person in a body of water comprising: means for generating an electrical transmission signal;   at least one pair of transducers disposed in a body of water including a transmitting transducer connected to receive the electrical transmission signal for launching a beam of acoustical waves in response to the electrical transmission signal in the body of water and a receiving transducer disposed opposite the transmitting transducer for receiving the acoustical waves and for converting received acoustical waves into an electrical received signal having a magnitude indicative of received acoustical wave intensity; and   means connected to the receiving transducer for producing a detected signal in response to the received signal, the detected signal having a magnitude indicative of the magnitude of the received signal, for producing a threshold signal having a magnitude varying in response to the magnitude of the detected signal, and for initiating an alarm when the magnitude of the detected signal falls below the magnitude of the threshold signal for at least a predetermined length of time.   
     
     
       22. The apparatus of claim 21 wherein the means for generating comprises a first oscillator for generating a modulating signal having a first frequency and a variable magnitude and a second oscillator connected to the first oscillator for generating a variable frequency signal having a frequency varying in response to the magnitude of the modulating signal as a swept frequency signal wherein the variable frequency signal has a higher frequency than the first frequency. 
     
     
       23. The apparatus of claim 22 wherein the means for generating comprises a bandpass filter receiving the variable frequency signal for eliminating undesired frequency components from the variable frequency signal and thereby producing the swept frequency signal. 
     
     
       24. The apparatus of claim 21 wherein the means for producing the threshold signal having a variable magnitude comprises a rectifier receiving the detected signal and producing a rectified detected signal and a low pass filter receiving the rectified detected signal and producing the threshold signal. 
     
     
       25. The apparatus of claim 21 wherein the means for initiating comprises means for comparing the magnitude of the detected signal to the magnitude of the threshold signal and for generating a reset signal pulse while the magnitude of the detected signal exceeds the magnitude of the threshold signal and means for detecting an interruption in reset signal pulses. 
     
     
       26. The apparatus of claim 25 wherein the means for detecting an interruption in the pulsed reset signal pulses comprises a timer for timing an interval between consecutive reset signal pulses. 
     
     
       27. The apparatus of claim 25 wherein the means for comparing comprises a comparator receiving the threshold signal and the detected signal. 
     
     
       28. An apparatus for detecting a person in a body of water comprising: means for generating an electrical swept frequency signal having a frequency that continuously varies between upper and lower limits;   at least one pair of transducers disposed in a body of water including a transmitting transducer connected to receive the swept frequency signal for launching a beam of acoustical waves in response to the swept frequency signal in the body of water and a receiving transducer disposed opposite the transmitting transducer for receiving the acoustical waves and for converting received acoustical waves into an electrical received signal having a magnitude indicative of received acoustical wave intensity; and   means connected to the receiving transducer for producing a detected signal in response to the received signal, the detected signal having a magnitude indicative of the magnitude of the received signal, for producing a threshold signal having a magnitude varying in response to the magnitude of the detected signal, and for initiating an alarm when the magnitude of the detected signal falls below the magnitude of the threshold for at least a predetermined length of time.